1. Field of the Invention
The present invention relates to protecting predetermined addressable data stored in programmable memory devices. More particularly, the invention relates to providing non-volatile memory sectors in conventional memory devices, for example, providing non-volatile "boot block" sectors in an industry standard, programmable memory devices such as electrically erasable read-only memory (EEPROM).
2. Description of the Prior Art
The use of programmable read only memory (PROM) is common in many digital electronic applications, including computers, cellular telephones, and video games. The advantage of a PROM is that the software programmed into the devices is non-volatile and available at any time. However, if the need arises to change the software codes programmed into the PROM, the system must be disassembled, the device removed and a new device installed.
The disadvantage of the PROM device has, in recent years, been overcome to some extent by the electrically erasable programmable read-only memory (EEPROM). This device provides the advantage of in-circuit re-programmability, allowing code changes to be made without removing the device, or even disassembling the system chassis. However, by allowing a method of electrically erasing and reprogramming the device by a program running on the host system, its ability to be totally non-volatile and impervious to disastrous inadvertent erasure has been comprised.
For example, if the system power was temporarily lost during the process of reprogramming the EEPROM for routine system software maintenance, the EEPROM would be left in a partially erased and unusable state. This condition would render the system inoperable, and disassembly and replacement would likely be required. Furthermore, there are malicious programs called computer viruses in wide circulation. If a user of a computer were to unknowingly run such a program, one designed to take advantage of a fully volatile EEPROM by destroying or modifying the system software stored on it, the damage might not be limited to an unusable EEPROM. The user's own data stored in other subsystems of the computer would also be vulnerable to the unauthorized program stored by the virus in the EEPROM.
Some new devices, commonly called boot block sectored EEPROMs, also referred to as flash ROMs, provide the reprogrammability of the EEPROM, and also a limited amount of safe, totally non-volatile PROM. These devices allow the safe storage of a small block of disaster recovery software, such that in the event that an error occurred during in-circuit reprogramming, the system could bootstrap itself to a minimum level required to recover from the error. These devices typically have a predefined protected memory area size, which limits their flexibility in some applications.